While it may seem like Linux-only projects are betraying their loyal base by developing Windows or OSX versions, I would argue that cross-platform development is actually better for Linux as a whole, better for individual software projects and their developers, and ultimately better for Linux users.

Generally I would agree with there being benefits in doing software development in a portable fashion.

For example I totally agree with shotsman that just having a broader range of tools available already makes it a worthwhile effort.

So in principle I would have expected to agree with the article's contents as well, but I don't.

First, the author is very inconsistent in their definition of Linux-only.
At first one would assume that the meaning is software only available on Linux, but one discoveres pretty quickly that Linux-only seems to mean not available on Windows/OSX.

Closer to the end of the article even this is not true anymore, Linux-only now meaning more or less Windows/OSX versions not being in total sync with Unix platforms.

Very sloppy definition of "only" IMHO.

The author then goes on to claim that having the software available for the two main proprietary desktop platforms would automatically increase project contributions (either as work or financially).

But then totally destroys that argument himself by claiming that he would totally seriously love to contribute to Calligra (as a developer, no less) but cannot because the Windows version is not released consequently enough.
Right!

Most projects have a majority of their developers using one platform.
Some projects might reach out to contributors on other platforms, e.g. by providing an initial port or even doing a release there now and then.
Some projects might not have the resoures to do that but would welcome any new contributors willing to do that.
Only very few projects will consciously avoid supporting platforms they deem to different than their main one and especially on Linux only a tiny fraction will deliberatly depend on a single platform's feautures.

So nice intent of advertising targetting multiple platforms but very inconsistent in execution.